Sulfonic acid group and/or carboxyl group-containing fluoropolymers have been recently receiving attention because of their use in applications such as electrolyte membranes for fuel cells or chemical sensors. In particular, fluoropolymers having a sulfonic acid salt group, such as —SO3Na, are used in ion exchange membranes for brine electrolysis.
The production of electrolyte membranes uses media for transferring a catalyst to the surface of electrolyte membranes. Conventional media mainly made of an organic solvent are desired to be replaced by aqueous dispersions of fluoropolymers for reasons of, for example, ease of handling. Since aqueous dispersions of fluoropolymers can be used on their own as coating compositions, they are suitable for film casting, impregnation and the like, and their applications are wide-ranging.
In the case where such aqueous dispersions are used for fuel cells or other electrochemical devices, impurities in the aqueous dispersions may have adverse effects on intended electrochemical reactions, for example. Therefore, there is a need for methods for removing such impurities.
One example is to perform an anion exchange treatment on a dispersion containing an ionic fluoropolymer having a group represented by —R1SO3− and an anion species that is not covalently bound to the ionic fluoropolymer (see, for example, Patent Literature 1). In EXAMPLES of Patent Literature 1, a dispersion B of a hydrolyzable fluoropolymer having —SO2F is cation-exchanged to remove Mn2+ ion, which can form MnO2 during hydrolysis, and then hydrolyzed, and the resulting product is contacted with a cation exchange resin at a temperature of 40° C. to 50° C., and then purified with an anion exchange resin.